Soft-lithography and micro-contact printing are tools of choice in contemporary research in areas such as, but not limited to, tissue engineering, microfluidics, and protein patterning on surfaces. Thin elastomeric films, such as, but not limited to, polydimethylsiloxane (PDMS) films, are used in a number of applications in soft lithography. The thin elastomeric films, such as polymer films, have several current applications such as, but not limited to, use in microfluidics, in cell and protein patterning, and as tissue engineering scaffolds. Thin polymer films prepared using soft lithography have several advantages over conventional microfabrication techniques including faster turnaround times, room-temperature processing, and lower overall cost.
Thin elastomeric films often need to be peeled from a substrate. Currently, the process of peeling the films from a substrate is a difficult manual process having low yield and requiring delicate manual manipulation. Unfortunately, manual peeling requires a great deal of skill and often causes damage to the films, leading to yields that are relatively low.
An example of elastomeric film fabrication and removal from a substrate is as follows. An elastomer, described herein as a PDMS polymer, for exemplary purposes, may be spin-coated on a uniformly flat or patterned glass or silicon substrate. The polymer is dried in an oven to let the polymer cure and turn into a solid film. When the peeled film is carefully peeled from the substrate, the film carries the negative of patterns on the substrate and can then be used for further downstream processes. When thin PDMS films are used in soft lithographic processes, it is necessary to peel the film off a silicon or glass master. Currently, the film is peeled from the mold by hand. By inserting a knife or the sharp point of tweezers under the edge of the film, the operator frees a small portion of the film from the surface so that it can be gripped by hand. FIG. 1 is a schematic diagram illustrating the prior art manner of providing for peeling of the film from the surface. The film is then held by hand and peeled off delicately. Unfortunately, as mentioned above, this manual action is difficult to perform without tearing the film or otherwise damaging it, and film damage is common. Success in peeling depends very much on the dexterity and the physical skill of the operator.
Even after a portion of the film has been peeled off the substrate, thin PDMS films tend to tear from the periphery. An edge bead formed during spin-coating is one important cause of such damage. The edge bead results from overflow of the polymer over the edge of the substrate. Upon curing, the overflow region locks the film around the edge and prevents uniform separation as the peeling process proceeds. This uneven separation causes tearing. To avoid such tearing, the substrate edge is scraped manually. Depending on the speed of spin-coating, the edge bead could be twice as thick as the PDMS film being peeled. The comparatively thin PDMS film would then be unable to provide sufficient strain energy for the edge bead to peel, resulting in tearing.
It should also be noted that when peeling a thin PDMS film, the overall geometry of the substrate affects the actuation that needs to be applied. Specifically, when peeling a film over a silicon wafer, the applied force must increase to account for the gradually increasing width of film. Once more than half the film has been peeled, the amount of force required to peel starts decreasing. If the operator does not diligently adjust her effort accordingly, the film peels off in an uncontrollable fashion and could tear.
FIG. 2 is a schematic diagram illustrating force actuation vs. peel length during peeling a film from a substrate, such as a circular wafer. As is illustrated by FIG. 1, from point A to point B, force increases as the width of film being peeled increases. From point B to point C, the width of film decreases and potentially causes instability.
Repeatability and efficiency are two key attributes of production-ready manufacturing processes. As mentioned above, the peeling process is not very repeatable. In fact, the physics of peeling are complex. Two key insights that can be drawn with regard to peeling include that peeling depends on a number of material properties such as, but not limited to, the surface energy of PDMS with regard to Silicon, and the Youngs modulus of PDMS, and geometric criteria such as, but not limited to, the angle of peeling and the method of peeling.
With the rapid development of PDMS in both experimentation and as a potential mass manufacturing technique, peeling is likely to be a bottleneck to the manufacturing process. An example of current process for preparing of PDMS includes the following and does not yield repeatable product. The process begins when the substrate is cleaned to remove organic contaminants, dried using nitrogen and coated with a monolayer of silane to reduce the adhesion between PDMS and silicon dioxide. A common method for preparing a PDMS sample is using a monomer and cross-linker in 10:1 ratio. The mixture of monomer and cross-linker is degassed and spin-coated on the substrate. The spin-coating rpm and the duration are chosen based on the final thickness desired. For example, spin-coating at 1000 rpm for 60 seconds results in a film about 100 μm thick. The sample is then baked in an oven (e.g., at 85° between 20 mins and a few hours). Unfortunately, variations in these steps and process parameters greatly affect the material property of the PDMS film. Even for the same preparation method, the material properties of the PDMS film are not repeatable. The modulus of elasticity of the film is very sensitive to the cross-linking ration. The final film thickness varies over the entire area of the substrate and is highly sensitive to changes in the spin-coating rotations per minute (rpm) and duration. Finally, it is difficult to repeatably create films of the same thickness.
Thus, a heretofore unaddressed need exists in the industry to address the aforementioned deficiencies and inadequacies.